The present invention relates generally to communication systems, and more particularly to communication systems such as wireless time division multiple access (TDMA) systems which utilize differential quadrature phase-shift-keyed (DQPSK) modulation or other types of phase modulation.
A phase modulation technique known as xcfx80/4 DQPSK is utilized to transmit digital data in certain types of communication systems, such as wireless TDMA systems. In accordance with this technique, data is transmitted by changing the phase of a modulated signal. Each phase shift, over a specific period of time, is referred to as a symbol. The technique achieves an increase in spectral efficiency by multiplexing two signals in phase quadrature. The two signals, an in-phase (I) signal and a quadrature (Q) signal 90xc2x0 out of phase with the I signal, are modulated onto a carrier signal to form a QPSK signal suitable for transmission. In the case of xcfx80/4 DQPSK, the four possible phase shifts are xc2x1xcfx80/4 (xc2x1/45xc2x0) and xc2x13xcfx80/4 (xc2x1135xc2x0), and a typical symbol period T in a conventional IS-136 or IS-54 wireless TDMA system is 41.2 xcexcs.
A conventional xcfx80/4 DQPSK demodulator suppresses the carrier signal and recovers the I and Q signals. The I and Q signals are sampled at intervals of T/4 and digitized using an analog-to-digital (A/D) converter. The digitized samples are then processed in a digital signal processor (DSP) to recover the phase of the symbol and its signal strength. FIG. 1 illustrates the T/4 sampling process for a given I or Q signal. The I or Q signal includes a stream of symbols, denoted Nxe2x88x921, N, N+1, N+2, etc. in this example. Each of the symbols of the I or Q signal is sampled at intervals of T/4, as shown.
In general, between a base station and a mobile unit in a wireless TDMA system, the T/4 sampling of symbols in a DQPSK demodulator is generally asynchronous with respect to the transmitted symbol. The best case situation, illustrated in FIG. 2, is when the four T/4 samples for a given symbol are taken during the most stable portion of the current symbol, i.e., symbol N. The worst case, illustrated in FIG. 3, occurs when one of the T/4 samples is taken at the transition between the current symbol N and a previous symbol Nxe2x88x921 or a subsequent symbol N+1. When taking samples asynchronously, one can guard against the worst case by comparing the quality of an odd sample pair, i.e., samples 1 and 3 in FIG. 3, with that of an even sample pair, i.e., samples 2 and 4 in FIG. 3, in every set of four T/4 samples, and keeping only the best pair for each symbol. This approach ensures that there will be at least two acceptable samples per symbol of the four T/4samples taken in the symbol interval, thereby guaranteeing an effective T/2 sampling quality. Although a similar approach could be used with a higher T/8 sampling rate in order to ensure an effective T/4 sampling quality, a more powerful A/D converter and DSP will generally be required, thereby increasing the complexity and cost of the demodulator.
A need therefore exists for a phase demodulation technique which can better synchronize the sampling process to the received symbols, such that improved performance can be obtained relative to the above-described conventional techniques, without significantly increasing the complexity and cost associated with the demodulation process.
The invention provides improved phase demodulation techniques for use with quadrature phase-shift-keyed (QPSK) signals and other types of phase-modulated signals in a communication system. These phase demodulation techniques utilize sample timing which is based at least in part on frequency information generated by frequency demodulating the phase-modulated signal. In an illustrative embodiment of the invention, a phase-modulated signal is separated into first and second portions. The first portion is then phase demodulated to generate demodulated symbols, while the second portion is frequency demodulated to generate a measure of the instantaneous frequency of the phase-modulated signal. The instantaneous frequency measure is then processed to identify one or more symbol transitions, and the identified transitions are used to establish the sample timing such that proper sampling of the symbols is ensured. For example, the measure of the instantaneous frequency of the phase-modulated signal may be a signal having a signature associated with a particular synchronization word utilized in the system, and may be processed to generate information which is used in a digital signal processor (DSP) or other suitable processing circuitry to generate, adjust or otherwise control a sample clock used in sampling the demodulated symbols.
By ensuring proper symbol timing in the demodulation process, the invention provides significantly improved bit error rate (BER) performance for the received data. For example, in a system utilizing xcfx80/4 DQPSK modulation, the invention can ensure a desired T/4 symbol sampling without any increase in the sampling rate or the complexity and cost of the demodulator. Although the invention is particularly well suited for use in applications such as wireless TDMA systems, it can provide similar advantages in numerous other communication system applications.